sustainability Article Long Range Transport of Southeast Asian PM2.5 Pollution to Northern Thailand during High Biomass Burning Episodes Teerachai Amnuaylojaroen 1,2,*, Jirarat Inkom 1, Radshadaporn Janta 3 and Vanisa Surapipith 3 1 School of Energy and Environment, University of Phayao, Phayao 56000, Thailand; [email protected] 2 Atmospheric Pollution and Climate Change Research Units, School of Energy and Environment, University of Phayao, Phayao 56000, Thailand 3 National Astronomical Research Institute of Thailand, Chiang Mai 53000, Thailand; [email protected] (R.J.); [email protected] (V.S.) * Correspondence: [email protected] or [email protected] Received: 14 October 2020; Accepted: 10 November 2020; Published: 2 December 2020 Abstract: This paper aims to investigate the potential contribution of biomass burning in PM2.5 pollution in Northern Thailand. We applied the coupled atmospheric and air pollution model which is based on the Weather Research and Forecasting Model (WRF) and a Hybrid Single-Particle Lagrangian Integrated Trajectory Model (HYSPLIT). The model output was compared to the ground-based measurements from the Pollution Control Department (PCD) to examine the model performance. As a result of the model evaluation, the meteorological variables agreed well with observations using the Index of Agreement (IOA) with ranges of 0.57 to 0.79 for temperature and 0.32 to 0.54 for wind speed, while the fractional biases of temperature and wind speed were 1.3 to 2.5 ◦C and 1.2 to 2.1 m/s. Analysis of the model and hotspots from the Moderate Imaging Spectroradiometer (MODIS) found that biomass burning from neighboring countries has greater potential to contribute to air pollution in northern Thailand than national emissions, which is indicated by the number of hotspot locations in Burma being greater than those in Thailand by two times under the influence of two major channels of Asian Monsoons, including easterly and northwesterly winds that bring pollutants from neighboring counties towards northern Thailand. Keywords: PM2.5; biomass burning; long-range transport of PM2.5; source of PM2.5 1. Introduction Air pollution is a widespread problem that affects human health and other atmospheric aspects, i.e., it can contribute to the warming of the atmosphere and can affect rain and cloud patterns. Air pollution is released from a number of man-made and natural sources including fossil fuels burning in electricity production, transportation, industry and households, agriculture, and waste processing. According to a 2014 World Health Organization (WHO) report the premature deaths of about 7 million people worldwide were caused by air pollution [1]. It is estimated that in developing countries approximately 300,000 to 700,000 people can be saved from premature death if aerosol levels are reduced to a safe level (an Air Quality Index (AQI) number under 100 signifies good or acceptable air quality) [2]. Southeast Asia is a region with frequent air pollution problems every year, particularly at the beginning of the year, from January to April. Biomass burning strongly dominates air pollution from the regional to local scale in Southeast Asia [3]. Moreover, severe haze events in this region caused by particulate pollution have become more intense and frequent in recent years. Widespread biomass burning occurrences and particulate pollutants from human activities other than biomass burning Sustainability 2020, 12, 10049; doi:10.3390/su122310049 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 10049 2 of 14 play important roles in degrading the air quality in Southeast Asia [4,5]. In addition, appropriate meteorological and topographic effects are also favorable conditions that contribute to the air pollution problem in Southeast Asia. In northern Thailand, all these factors are combined. Most cities in northern Thailand are located in the mountainous area and surrounded by paddy fields. Larger villages like Chiang Mai face increasing problems due to traffic jams, but farmers also burn stubble in preparation for the coming rain and rice planting at this time of the year, and these narrow valleys provide perfect bowls for this smog and smoke. The contributing pollutants in Northern Thailand, however, are not only from national sources but also from long-distance air pollutants introduced by meteorological factors, i.e., wind, temperature, and humidity [6–9]. The climate of northern Thailand is characterized by monsoons. The period from mid-February until the end of May is the transition period from Northeast monsoon (most prevalent in December–January) to Southwest monsoon (most prevalent in July–August) climates. The hottest weather observed during March–April, coinciding with the presence of intensive thermal lows in the area [10]. During the transition period, winds can transport air pollutants from the surrounding area into northern Thailand [11]. The northeastern monsoon transfers air pollutants from China to the southern Chinese sea from mid-December to mid-April and can then flow continuously to the mainland Southeast Asia. Exposure to high levels of air pollution can cause a variety of adverse health outcomes. PM2.5 is the most important air pollutant and strongly affects human health. Recent calculations of global premature mortality rates on the basis of high-resolution global O3 and PM2.5 models show that Southeast Asia and the Western Pacific account for around 25% and 45% of world mortality [12,13]. PM2.5 can penetrate deeply into the respiratory tract and enter the lungs. Exposure to small particles can also impair the function of the lungs and exacerbate medical conditions such as asthma and heart disease [14–16]. At the beginning of 2020, the PM2.5 measurements of air pollution in Chiang Mai, one of the largest cities in northern Thailand, reached a staggering level of 330 on the PM2.5 concentration over the weekend, making the northern city the most polluted city in the world, while the airborne pollution levels across Thailand’s northern region vary between 100 and 390 of the air pollutants concentrations, according to AirVisual data. There have been a few studies exploring the source contributions of PM2.5 in this region. For example, Chueinta et al. [17] reported the characterization and source identification of fine and coarse particles collected in urban and suburban residential areas in Thailand and later performed an extended study on the Bangkok metropolitan curbside [18]. Leenanupan et al. [19] carried out similar work on the characterization of fine particulate pollution in the Mae Hong Son province in the north of Thailand. A few collaborative studies on fine and coarse particulate air pollution at the Asia Pacific regional scale were also reported, e.g., Oanh et al. [6,7]; Ebihara et al. [20,21]; Hopke et al. [22]. Moreover, only a few long-term PM2.5 and PM10–2.5 monitoring data are available for elsewhere in this region. However, it is unclear how much neighboring countries contribute to the air pollution problem in northern Thailand. In addition, Lee et al. [5] found that nitrate aerosol was the major component of PM2.5 particles in Southeast Asia. This work applied atmospheric model coupling with the air pollution model to investigate the potential contributions of biomass burning in air pollution in Northern Thailand. We used the Weather Research and Forecasting (WRF) model version 3.8.1 to simulate the meteorological conditions on March 2016. The model’s initial and boundary conditions were taken from the Final Analysis Data (FNL) [23], and the modeled temperature and wind speed were compared to a dataset from the Pollution Control Department (PCD). To clarify the model’s capabilities, statistical analyses such as Index of Agreement (IOA) and Fractional Bias FB were used for the model evaluation. The output from the WRF model was used as the meteorological conditions to be inserted into the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model to determine the long-range transport of regional air pollutants from the neighboring countries of Southeast Asia toward northern Thailand. SustainabilitySustainability 20202020, 12,, x12 FOR, 10049 PEER REVIEW 3 of 143 of 15 We use a coupled atmospheric model called the WRF model version 3.8 [24] to generate the meteorological2. Materials input and Methods for HYSPLIT, which is an air quality model. The results from HYSPLIT were then usedWe to useidentify a coupled the PM atmospheric2.5 pathway model in Southeast called the Asia. WRF model version 3.8 [24] to generate the meteorologicalThe WRF model input was for HYSPLIT,developed which to study is an air several quality atmospheric model. The results factors from and HYSPLIT also to were be thenused for operationalused to identifyweather the forecasting. 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